1. Field of the Invention
The invention generally relates to generation of differently phased oscillator signals for use in harmonic rejection mixers (HRMs). More particularly, the invention relates to providing HRMs with an arbitrarily large number of differently-phased local oscillator (LO) signals to reject substantially more harmonics (“images”) than conventional arrangements, especially using LO signals that are not dependent on phase lock loop (PLL) frequency dividers to divide a high-frequency signal to a suitable LO frequency.
2. Related Art
Harmonic-rejection mixers (HRMs), which are designed to reject local oscillator (LO) harmonics (or “images”), are known in the art. See, for example, “A 1.75 GHz Highly-Integrated Narrow-Band CMOS Transmitter with Harmonic-Rejection Mixers” Weldon, J. A.; Rudell, J. C.; Lin, L; Narayanaswami, R. S.; Otsuka, M.; Dedieu, S; Tee, L.; Tsai, K-C; Lee, C-W; and Gray, P. R.; Section 10.4 of Digest of Technical Papers of the 2001 IEEE International Solid-State Circuits (ISSC) Conference, 5–7 Feb. 2001, pages 160–162 (hereinafter called “the Weldon et al. reference”). FIG. 1 shows part of FIG. 10.4.1 from the Weldon et al. reference.
In FIG. 1, orthogonal (quadrature) I and Q signals pass through respective digital-to-analog converters (DACs) and low pass filters (LPFs) to reach four harmonic rejection mixers (HRMs). Also input to the HRMs are local oscillator (LO) signals φ1, φ2, φ3, φ4, provided from a phase divider. The input to the phase divider is provided by a phase lock loop (PLL) that runs from a crystal oscillator XTAL and has a much higher frequency than the LO frequency. Within the PLL, a first frequency divider D1, a phase difference detector Δ, a voltage controlled oscillator VCO, and a feedback frequency divider D2, are provided in a conventional PLL arrangement. Outputs of the four HRMs are subtracted or added, pair-wise, to provide intermediate frequency (IF) signals that are subsequently subjected to further mixing and summing, downstream.
Undesirably, conventional square wave LOs generate a significant amount of odd harmonics. For example, if a LO has a fundamental frequency of 100 MHz, then significant harmonics are generated at odd multiples thereof, namely, at 300 MHz, 500 MHz, 700 MHz, and so forth. Unfortunately, these harmonics mix with the modulated information signals. Harmonics that are close in frequency to the fundamental frequency are difficult to filter or otherwise eliminate, using conventional techniques.
Weldon et al. generate four-phase LOs (two pairs of orthogonal signals) and sum the LO signals of different phases. In this manner, Weldon et al. creates a very rough “stair step” approximation of a sin wave at the LO output. Weldon's FIG. 10.4.2 shows the details of an HRM that performs this rough “stair step” approximation. In providing even this rough “stair step” approximation, Weldon et al. reduce the magnitude of some lower-order harmonics, as compared to a conventional purely square wave LO. The Weldon et al. reference states that their arrangement significantly rejects the third and fifth harmonics (−68 and −69 dB, respectively).
In the Weldon et al. reference, as in many conventional arrangements, the generation of multiple phases φ1, φ2, φ3, φ4 has been achieved using a phase lock loop, or a combination of a PLL and a divider. Unfortunately, such conventional arrangements severely limit the number of phases available for a particular LO frequency. In Weldon's example, only four phases are produced at the LO output, substantially limiting the closeness with which a LO output can emulate a sinusoidal output, thus frustrating rejection of higher harmonics.
Because of difficulties in filtering out “close” (lower-order) harmonics, especially in modern systems having increasingly higher fundamental frequencies, there is a need in the art to reject not only the third and fifth harmonics, but also the seventh, ninth, and further harmonics. This need could be fulfilled by providing an HRM output that more closely emulates a sinusoidal signal, but with the limited number of LO phases available in conventional arrangements, this need has not been fulfilled. Accordingly, there is also a need in the art to generate an arbitrarily large number of LO phases, based on orthogonal I and Q signals, to ultimately allowing higher-order harmonics to be rejected in harmonic rejection mixers.
Furthermore, Weldon's conventional 4-phase generation method is not applicable for all possible LO frequencies. That is, the PLL frequency needs to be a multiple of the desired LO frequency, a requirement that limits the applications that can use the HRM. Thus, there is a further need in the art to provide an arrangement for generating a large number of LO phases, at a frequencies that can be flexibly chosen rather than being limited to a limited number of frequencies.